Three-dimensional (3D) reconstructions are being employed increasingly frequently in medical X-ray-based imaging. This involves reconstructing a 3D volume from a plurality of projection images from various directions or angulations, which is to say in different positions. This is classically carried out on special computed tomographs, but in the meantime it is also the state of the art on so-called C-arm machines. In the case of such a procedure, a so-called rotational scan, attention is paid primarily, in terms of a patient's dose uptake, to an actual total dose, that is to say the dose, in particular the cumulative dose, that primarily impacts the patient's internal organs. This total dose is responsible for possible stochastic risks. In other words, a raising of the total dose results in an increase in the risk of mutations and uncontrolled cell growth, commonly known as “cancer”. Deterministic damage at a skin entry point, as is attended to primarily in the case of classical two-dimensional (2D) projection imaging, is less significant in this case since the radiation entry zone is constantly being changed due to the switching into different positions.
According to the present-day state of the art, most X-ray machines now have automatic exposure regulation. An exposure regulation system may set five parameters: a tube voltage, a tube current, the exposure time per image or position, a prefiltering parameter, and an emitter size or a focus. In this regard, the selection of the parameters strongly influences the accumulating total dose for the patient, and also the resultant image quality both for two-dimensional projection images and also a three-dimensional reconstruction. In this regard, the influence of the current and exposure time is quantifiable: raising these has a linear effect on actual dose and image quality. The image quality may be described, for example, by the square of the signal-to-noise ratio (SNR) or a contrast ratio. Raising the X-ray voltage, on the other hand, increases the actual dose, or the actual individual dose per position and also the actual total dose, more than proportionally since deeper layers may be reached and higher quanta of energy are used. The effects of this parameter on image quality, on the other hand, are strongly dependent on the conditions. Furthermore, an important factor for image quality is of course the total number of quanta in fact transmitted and reaching a detector or image receiver forming part of the computed tomograph.
DE 197 37 408 A1 describes a method and a device for modulating an X-ray current in a computed tomography system. In this regard, a desired noise level for a final image is selected, and a desired minimal X-ray photon measured value and also a desired average X-ray photon measured value identified, with which an image corresponding to the desired noise level may be generated. During a scan, prevailing X-ray photon measured values are compared with the desired average X-ray photon measured values at different projection angles and used to generate an X-ray modulation factor. This modulation factor is then used to modulate the X-ray tube current.
U.S. Pat. No. 6,507,639 B1 describes a method for modulating a radiation dose of an X-ray tube, which is dependent on an angle of rotation of a drum bearing on which the X-ray tube is arranged. In this regard, the modulation is carried out such that an instantaneous intensity of the X-ray radiation likewise reaches its maximum at the time of a maximal absorption of the X-rays by a patient and not at some other time.
DE 10 2005 021 020 A1 describes a method for calculating an orthogonal X-ray attenuation of an object supported on an object table of a computed tomograph on the basis of a measured reference X-ray attenuation. For the purpose of saving on X-ray dose, the projection angle-dependent fluctuation in the attenuation is partly offset by configuration of the radiation intensity generated by the X-ray emitter.
U.S. Pat. No. 5,379,333 A describes an X-ray computed tomograph in which a current of an X-ray tube is modulated as a function of a drum bearing angle in order to reduce a total dose for the patient without significantly raising the image noise in the process. To do this, test pictures are made and the absorption of the X-ray radiation by the patient determined in two orthogonal directions. A modulation profile is then calculated, according to which the X-ray current is modulated during image capture, in which the X-ray tube is rotated around the patient. In this regard, the modulation may be effected according to a sine or a cosine wave.
The article “CT Dose Reduction and Dose Management Tools: Overview of Available Options,” by Cynthia H. McCollough, Michael R. Bruesewitz, and James M. Kofler in the journal RadioGraphics, 2006, Vol. 26, pp. 503-512, provides an overview of opportunities for lowering an X-ray dose in computed tomography. The possibility, inter alia, of varying an X-ray current while the X-ray tube is rotating around a patient is examined in this regard. Starting with a constant tube current, the current is modulated sinusoidally in this process.